Invariants of singular sets of smooth maps

Abstract

A singular point of a smooth map F: M -> N of manifolds is a point in M at which the rank of the differential dF is less than the minimum of dimensions of M and N. The classical invariant of the set S of singular points of F of a given type is defined by taking the fundamental class [\bar{S}]\in H_*(M) of the closure of S. We introduce and study new invariants of singular sets for which the classical invariants may not be defined, i.e., for which \bar{S} may not possess the fundamental class. The simplest new invariant is defined by carefully choosing the fundamental class of the intersection of \bar{S} and its slight perturbation in M. Surprisingly, for certain singularity types such an invariant is well-define (and not trivial) despite the fact that \bar{S} does not possess the fundamental class. We determine new invariants for maps with Morin singularities---i.e., singularities of types A_k for k>0 in the ADE-classification of simple singularities by Dynkin diagrams---and, as an application, show that these invariants together with generalized Miller-Morita-Mumford classes form a commutative graded algebra of characteristic classes that completely determine the cobordism groups of maps with at most A_k-singularities for each k>0.